This invention relates generally to measuring intracranial pressure in patients, and more particularly to measuring intracranial pressure by non-invasively monitoring the pulsatile components of cerebrospinal fluid contained within the head.
Intracranial pressure (ICP) is an important parameter in the management of closed head trauma. For example, head trauma can cause edema which leads to increased ICP as well as decreased brain compliance. High ICP must therefore be treated aggressively in order to prevent secondary neurological damage. Conditions, other than trauma, which can lead to elevated ICP include intracerebral hematoma, central nervous system infections, subarachnoid hemorrhage, space-occupying lesions, increased gravitational forces and whole body acceleration. When ICP is elevated, it can vary widely from moment to moment and dynamic measurement of ICP is extremely useful. Dynamic measurement of ICP may also provide evidence of hypertension before the onset of clinical signs and symptoms.
Most current methods for measuring ICP are invasive, either utilizing an intraventricular catheter, a subarachnoid screw, an epidural pressure sensor or a sound wave burst into the inner ear. The leading complications of invasive ICP monitoring are insertion-related hemorrhage and hematoma formation, acute overdrainage of cerebrospinal fluid, and infection. An example of the generation of a sound wave causing the bones of the middle ear to react and transfer pressure to the eardrum can be found in U.S. Pat. No. 4,841,986 issued to Marchbanks. In addition to the extreme discomfort to the patient resulting from such a method, the results are not very accurate.
Although other methods claim to non-invasively measure ICP, they either measure some physiological quantity that has no fixed relationship to ICP or the relationship only allows for static ICP measurements accompanied by fairly complex calibration schemes. A method described by U.S. Pat. No. 5,617,873 issued to Yost and Cantrell, describes the non-invasive use of a constant frequency pulsed phase-locked-loop (CFPPLL) ultrasonic device to statically measure ICP. While such an ultrasonic technique is painless and effective in reducing the invasiveness of the ICP measurement, other problems are left unsolved. The CFPPLL receives a reflected toneburst signal from the front of the head and phase compares the reflected signal to a reference signal. The difference in phase between the two signals is attributed to changes in cerebrospinal fluid volume which is due to ICP changes. The phase comparison results in a control voltage which represents the amount of voltage required to bring the two signals into quadrature. A pressure-volume index along with a fairly complex calibration scheme utilizing either a tiltable bed or pressurized cap allows for the conversion from control voltage to cerebrospinal fluid volume, to ICP. The limitation associated with such a method, is that the ICP measurements are relatively static because they depend on changes in the cerebrospinal fluid volume, a relatively static physiological quantity. Furthermore, the device as described lacked sufficient responsiveness to measure dynamic quantities. The calibration scheme is also sufficiently complex to limit the clinical usefulness of the method.
Accordingly, one object of the invention is to non-invasively measure ICP within a patient""s head.
A further object of the invention is to measure ICP by using a non-invasive ultrasonic measurement device.
A further object of the invention is to monitor physiological quantities such as pulsatile components which enable accurate, dynamic ICP measurements to be made.
Another object is to use a measurement device with sufficient responsiveness to monitor dynamic quantities.
Still another object is to allow measurement of ICP with a relatively simple calibration scheme.
Additional objects and advantages of the present invention are apparent from the drawings and specification which follow.
The present invention monitors one or more blood pressure pulsatile components present in the brain as a method of measuring ICP. Examples of these pulsatile components are systolic and diastolic blood pressures which are partially transferred to the cerebrospinal fluid (CSF) contained in the head. Overall blood pressure is maintained by the complex interaction of the homeostatic mechanisms of the body and is moderated by the volume of the blood, the lumen of the arteries and arterioles, and the force of the cardiac contractions. These cardiac contractions transfer arterial pulses to the blood vessels in the brain tissue and the membrane surrounding the CSF. The blood vessels in turn partially transfer the arterial pulses to the adjacent CSF. Continuous monitoring of these pulses allows ICP measuring because as the ICP varies, the pulsatile components of the CSF also vary. Capitalizing on this direct relationship for the purpose of non-invasively measuring ICP is not found in the prior art.
The present invention generates an acoustic signal on one side of the head which is reflected back from the other side of the head. As the reflected acoustic signal travels through the brain, pulsatile variations in the CSF cause associated phase variations in the signal. The preferred embodiment of the present invention utilizes the phase shifting capabilities of the CFPPLL described by U.S. Pat. No. 5,214,955 issued to Yost and Cantrell to monitor this variation in the CSF pulsatile components, which patent is herein incorporated by reference, as if set forth in its entirety. After receiving the phase varied acoustic signal and converting it into a phase varied electrical signal, the CFPPLL phase compares the electrical signal to a reference signal, generates an error signal in relation to the difference in phase between the two signals, integrates the error signal into a control voltage, and shifts the phase of the reference signal based on the control voltage such that the two signals are placed in quadrature.
By integrating the error signal into a measurement voltage with filtering circuitry having appropriate responsiveness to biological systems, a signal can therefore be sampled and compared frequently enough to dynamically monitor pulsatile components. Such circuitry simply requires the appropriate time constant and is commonly known in the field. The present invention also employs a relatively simple calibration method to convert the pulsatile components into ICP with a measurement baseline. The method involves tilting the head to a new position to cause a change in ICP of a known amount. The tilt also causes a change in the CSF pulsatile components, which causes a change in the error signal and associated measurement voltage. The change in known ICP divided by the associated change in measurement voltage allows a measurement baseline to be obtained and therefore permits monitoring of the pulsatile components associated with subsequent changes in ICP. Since respiration pulses also affect intracranial pressure, a system which monitors blood gases can be used as an alternative calibration method should tilting the head be deemed undesirable.
Further, another possible calibration method to account for changes in the pulsatile components caused by changes in blood pressure could be achieved by correlating the change in measurement voltage with changes in blood pressure measured at a different point on the body, such as the wrist.